Electro-coating apparatus and method

ABSTRACT

In an high speed electro-coating apparatus having a plurality of electro-coating cells (96) arranged around the periphery of a rotatable turnable (14), each can (128) is placed upright on a cell lid (100) below a hollow cell body (94) and is introduced into that body (94) by upward movement of the cell lid (100). Separate simultaneous flows of electro-coating fluid fill and immerse the can (128), and each cell (96) receives electro-coating current pulses via slip-ring segments (272-278) carried on the turnable (14) past respective sets of brushes (286) which are disposed at the respective electro-coating stations. Control apparatus (300,302) is arranged to feed current to the brush sets only during the periods of turntable rotation during which each brush (286) has full contact with an adjacent slip-ring segment (278) and for as long as such full contact is maintained.

TECHNICAL FIELD

This invention relates to an electro-coating apparatus, and to a meansfor and a method of supplying electric current to the respectiveelectro-coating cells of such an apparatus.

BACKGROUND ART

Our British Pat. No. 2 085 474 describes a cell for electrophoreticallycoating a can. The cell comprises a hollow body which surrounds the can,a lid at the top which closes the cell, and a hollow mandrel disposedwithin the cell and so arranged that the can is held mouth downwards inequi-spaced relationship to the interior of the cell and the exterior ofthe mandrel. With an electrolyte, in the form of an electrophoreticcoating material, passing through the mandrel into the can, an electricpotential difference is applied between the can and the mandrel todeposit a coating on the interior of the can.

Our British Pat. No. 2 085 922 describes an apparatus for applying aseries of electric current pulses between an electro-coating cell and acan enclosed within that cell whereby to progressivelyelectrophoretically coat the can. The apparatus includes a rotatingturntable, a plurality of cells disposed around the circumference of theturntable, and a pair of segmented sliprings carried by the turntable.Each cell is electrically connected between corresponding segments ofthe two slip-rings. Respective sets of stationary brushes cooperate withthe two slip-rings at circumferentially spaced positions. Each brush setsupplies an electric current pulse of predetermined time duration toeach cell in turn as the turntable rotates. Thus, as the turntablerotates, and each cell is carried past the successive sets of brushes,that cell is energised by a succession of fixed duration current pulses.

In order to prevent arcing between adjacent slip-ring segments, adjacentsegments are spaced apart by spacers, and each current pulse is timed soas to start only after a slip-ring segment is in full contact with abrush set, and to continue for a predetermined time period such thatcurrent flow always ceases before full contact of the brush set with thesegment ceases.

As the duration of each current pulse is fixed, the proportion of theslip-ring segment travel from one brush set to the next during whicheach such current pulse flows depends on the rate of travel of theslip-rings past the brush sets, the maximum proportion being obtainedwhen the turntable speed is highest. Thus, the maximum turntable speeddetermines the duration of the current pulses, and the time periodswhich the electro-coating process is temporarily halted are short.

On the other hand, at low turntable speeds, current flows during only asmall proportion of that slip-ring segment travel. Consequently, at lowturntable speeds there are considerable time periods between successivecurrent pulses, during which the electro-coating process is halted.

It has been observed by the present inventors (a) that during theinter-pulse periods where the electro-coating process is temporarilyhalted, the electrical resistance of the coating so far depositedincreases with time, so that for a given applied voltage the rate ofdepositing additional coating material during the next pulse is reduced,and (b) that with that prior art apparatus, a desired coating weight isachieved more quickly when the turntable speed is higher than when thespeed is lower.

The present invention seeks to provide an apparatus in which (a) eachcurrent pulse flows for as long a time as possible regardless of theturntable speed, (b) the time periods during which the electro-coatingprocess is temporarily halted are minimised, and (c) the speed of theelectro-coating apparatus can be readily adjusted so as to match it tothe speed of other machines in a can production line without adverselyaffecting the performance of the electro-coating process in each cell.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention an electro-coatingapparatus for electro-coating a container comprises a turret for movinga plurality of cells each of which supports a container, a segmentedslip-ring of which each segment is operably connected to a cell, and aplurality of brushes for applying an electrical potential difference tosuccessive segments of the slip-ring so that as the slip-ring rotateseach segment in turn contacts a brush to collect a pulse of current fordelivery to the associated cell and container; which apparatus ischaracterised in that switching means discern the initial full contactof a brush and a segment and then apply a potential difference which ismaintained by said switching means until shortly before the brush isabout to break its full contact with the segment, so that substantiallythe whole length of each segment is used for current conduction at allturret speeds and the interval between current flows in the cell isminimal.

The present invention also extends to the methods of supplyingelectro-coating current to electro-coating cells, which methods aredescribed hereinbelow.

Other features of the present invention will appear from a reading ofthe description that follows hereafter and of the claims appended at theend of that description.

One electro-coating apparatus embodying the present invention, and itsmethod of operation, and various modifications of such apparatus andmethod (all according to the present invention), will now be describedby way of example and with reference to the accompanying diagrammaticdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In those drawings:

FIG. 1 shows a pictorial representation of the electro-coatingapparatus;

FIG. 2(a) shows diagrammatically a vertical cross-section taken on thediametral plane II--II (indicated in the FIG. 1) of a turntableincorporated in such apparatus;

FIG. 2(b) shows an enlarged vertical diametral cross-section takenthrough a cell closer which is shown in FIG. 2(a);

FIG. 3 shows diagrammatically a vertical cross-section taken on a radialplane III--III (indicated in the FIG. 1) of the turntable, and showingthe internal construction of one of several electro-coating cellscarried on that turntable;

FIG. 4 shows a partly sectioned pictorial view of one saidelectro-coating cell;

FIG. 5 shows diagrammatically the arrangement of a twin fluid-supplyvalve for controlling the supply of electro-coating fluid to one suchcell, FIG. 5(a) being a vertical cross-section through the valve, andFIG. 5(b) being a plan view of the valve;

FIG. 6 shows the construction of a slip-ring and brushgear assemblywhich constitutes the upper part of the apparatus shown in the FIG. 1,FIG. 6(a) showing a partly sectioned pictorial view of the assembly,FIG. 6(b) showing a scrap plan view of a pair of turntable rotationdetecting sensors incorporated in that assembly, and FIG. 6(c) showing ascrap vertical section revealing a turntable datum-position detectingsensor incorporated in that assembly;

FIG. 7 shows diagrammatically, in vertical sectional views, theconstruction of the slip-ring and brushgear assembly of FIG. 6, FIG.7(a) showing a slip-ring carrying part of that assembly, and FIG. 7(b)showing that slip-ring carrying part in association with a brushgearcarrying part of that assembly;

FIG. 8 shows pictorially the dispositions and external details of infeedand outfeed devices which form part of the electro-coating apparatus,FIG. 8(a) showing an enlargement of the lower part of the FIG. 1, andFIG. 8(b) showing a plan view of the infeed device;

FIG. 9 shows in a further diagrammatic form the subject matter shown inthe FIG. 8, FIG. 9(a) showing pictorially the infeed and outfeeddevices, and FIG. 9(b) showing pictorially a mechanism incorporated inthe outfeed device;

FIG. 10 shows in a vertical cross-section the construction and mode ofoperation of the infeed device shown in the FIG. 8, which cross-sectionis taken on a diametral plane of the infeed device;

FIG. 11 shows in a vertical cross-section the construction and mode ofoperation of the outfeed device shown in the FIG. 8, which cross-sectionis taken on a diametral plane of the outfeed device;

FIG. 12 shows the construction of one of several can gripper assembliesincorporated in the outfeed device, the respective FIGS. (a) to (d)showing respective views which are self-evident from those FIGS.;

FIG. 13 shows a diagrammatic vertical diametral cross-section of onehalf of the turntable and indicates thereon the flow path ofelectro-coating fluid which circulates within the apparatus when inoperation;

FIG. 14 shows a diagrammatic vertical diametral cross-section throughone of the electro-coating cells and indicates thereon the flow path ofelectro-coating fluid which circulates through the cell during theelectro-coating process;

FIG. 15 shows a diagram showing the cycle of events that occurs duringone revolution of the turntable;

FIG. 16 shows a schematic diagram showing the principal electric circuitcomponents involved in the control of the electro-coating current pulsesthat are passed through each cell during the process;

FIG. 17 shows the time sequence of current pulses that are caused toflow in one cell as it is carried through a series of electro-coatingstations by rotation of the turntable;

FIG. 18 shows an electric circuit diagram of the apparatus;

FIG. 19 shows schematically the turntable in relation to (a) variousassociated monitoring and control devices, and (b) various electricalmonitoring and control items and stages that are variously executed inhardware and/or software form; and

FIG. 20 indicates a series of graphs which relate electro-coatingcurrent and time, the respective graphs (a) to (d) demonstrating theadverse effect on the electro-coating process of increasing the durationof the time intervals between successive current pulses.

BEST MODES OF CARRYING OUT THE INVENTION

Referring now to the drawings, particularly to the FIGS. 1 to 2, theapparatus there shown comprises a static base structure 10* whichcarries on a bearing 12* a rotatable turntable or drum unit 14*. (Inthis specifation, an asterisk shown in association with a referencenumber indicates a first mention of that reference number.) The drumunit includes upper and lower annular plates 16*, 18* spaced apartvertically by an outer cylindrical wall 20*, a plurality ofcircumferentially-spaced, apertured, radial webs 22*, and an innercylindrical wall 24*.

The upper annular plate 16 carries centrally, for rotation therewith, asuperstructure 26* which includes lowermost a bearing plate 28* seatedon and secured to the inner peripheral parts 30* of the upper annularplate 16. That bearing plate supports centrally a bearing 32* whosepurpose will become apparent later, and above it a slip-ring unit 34*.The latter carries uppermost, on a central bearing unit 36*, abrush-gear unit 38* which shrouds the slip-ring unit, and which is heldagainst rotation by a torque arm 40* which engages a fixed post 42*carried on the base structure 10.

The lower annular plate 18 of the drum unit carries beneath it a toothedgear ring 44* with which a driving gear pinion 46* engages. A geareddriving motor 48* carried on the base structure 10 is coupled to, anddrives when energised, that gear pinion.

Secured within the inner cylindrical wall 24 of the drum unit 10 is afunnel member 50* which lies above and overlaps the outer peripheralparts of a stationary funnel member 52* which is secured to the upperpart of a cylindrical fluid collector pan 54* which is itself carriedaround its girth on supporting parts 56* formed on the base structure10.

The collector pan has formed in its base plate 58* a plurality of fluidexit holes 60* for enabling fluid collecting in the pan to flow away toa fluid reservoir tank 62*. That base plate also carries a centralcollar 64* to which is secured, on its lower side, the delivery pipe ofa pump 66* which is arranged to draw its intake fluid from the reservoirtank 62, and on the upper side thereof, the bottom end of avertically-disposed fluid supply tube 68*. That tube is steadied at itsupper end in, and extends through, the bearing 32 supported in thebearing plate 28.

Extending upwardly from that bearing plate and sealed around the bearing32 is an upwardly extending cylindrical wall 70* which is closed at itsupper end by a removable cover plate 72* and constitutes a stationaryfluid distribution chamber. A series of radial ports formed in thatcylindrical wall have secured therein the respective ends of a pluralityof fluid feed pipes 74*. The flow of fluid to those pipes is controlledby a stationary cylindrical baffle 76* which is secured to the upper endof the vertical supply tube 68 and which has formed in its cylindricalwall 78* a series of graduated ports 80* for effecting variation of theflow of fluid to the respective feed pipes 74 as the drum unit 14revolves around its vertical axis through successive predeterminedangular positions relative to the baffle.

The drum unit 14 also incorporates an intermediate annular plate 82*which is sealed to the cylindrical wall 20 and which carries externallyan upstanding wall 84*. A static cylindrical shield 86*, carried on thebase structure 10, overlaps at its upper end the periphery of the upperannular plate 16, and overlaps at its lower end the upper peripheralparts of the cooperating upstanding wall 84, so as to prevent the escapeof fluid from the annular region enclosed by the shield 86.

The upper, lower and intermediate annular plates 16, 18 and 32 of thedrum unit 14 have formed therein respective sets of thirty-twocircumferentially-spaced, circular apertures 88*, 90* and 92* (indicatedin the FIG. 2). Corresponding apertures of the respective sets aredisposed in vertical alignment one with another.

The upper annular plate 16 carries in each of its said apertures 88 avertically-dependent body portion 94* of an electro-coating cell 96*.The lower annular plate 18 supports in each of its said apertures 90 avertically-upstanding cell closing actuator 98* (referred to later as a"cell closer"), at the upper, movable end of which is carried a cellclosure member 100* (referred to later as a "cell lid").

The cell closer extends upwardly through the corresponding aperture 92formed in the intermediate plate 82, and is located there by thataperture, so that on energisation of the cell closing actuator 98 byhigh pressure air the cell lid 100 is raised into a position in which itabuts the lower end of the cell body 94 and so closes off and thuscompletes the cell 96.

Each cell closer 98 incorporates a cylinder 102* secured on theintermediate annular plate 82 (see FIG. 2(b)), in which cylinder acooperating elongated, tubular piston 104* is vertically movable. Thatpiston is urged upwardly by air supplied continuously to the cylinder102 at a suitable high constant pressure from a supply source (notshown) via a rotatable air coupling 106*, a distribution manifold 108*secured in the central upper part of the slip-ring unit 34, and a feedpipe 110*.

The respective cell closer cylinders 102 are all connected together byrespective interconnecting pipes 112*, each of which interconnects apair of adjacent cylinders. The ring-like pneumatic system so formed isconnected at four equi-spaced positions thereon, through four said feedpipes 110, with said distribution manifold 108, so that all of thecylinders are constantly energised by a supply of high pressure air.

In each cell closer 98, a tubular piston rod 114* is connected at itsupper end to a lid-receiving socket plate 115*, which is also secured tothe upper end of the elongated piston 104, and carries near its lowerend (remote from the cell lid) a transverse stud 116* on which iscarried rotatable cam follower wheel 118*.

A protective cylindrical shroud 119* secured at its upper end to thesocket plate 115 encircles the cylinder 104. The upper end of thatcylinder carries an electro-coating fluid excluding sealing ring 120*which cooperates with the shroud to enclose the space above that ring.Breathing apertures 121* are provided in the upper ends of the tubularpiston 104 and the tubular piston rod 114 so as to enable that enclosedannular space (confined between the upper end of the piston 104 and theshroud 119) to breath, on movement of the piston and piston rod in thecylinder, via the lower open end of the tubular piston rod 114, whichend communicates with a dry area of the apparatus.

Supported from the base structure 10 is an arcuate cam member 122* whichis positioned below the intermediate annular plate 82, and lies radiallyadjacent the lower parts of the respective cell closers 98, so as tocooperate with and position the respective cam follower wheels 118 asthe cell closers are carried around by the drum unit 14 into and througha predetermined range of angular positions relative to the basestructure 10.

The vertical depth of that cam member varies progressively in a mannersuch as to cause each cell closer piston rod 114 in turn, as the drumunit rotates through said range of angular positions, to be lowered (asa result of an increasing depth of the cam member and in opposition tothe biassing force provided by the high pressure air supplied to thecell closers) to its lowermost position, and then to be raised again toits uppermost position under the biassing action of the high pressureair supplied to the cell closers and in accordance with a decreasingdepth of the cam member.

Referring now to the FIGS. 3 and 4, each cell lid 100 includes acircular can-support plate 123* and a plurality of circumferentiallyspaced, non-conducting pins 124* projecting upwardly from thatcan-support plate. The upper surface 126* of the can-support plate has acontour which closely complements that of the bottom wall of a can 128*that is to be electro-coated in the cell. The pins 124 are intended toengage and act as minimal base supports for a can 128 to be coated.

The can support plate 123 is carried in a recess formed in a lidsupport/supply member 130*, and is retained therein by a clamping ring132*. A resilient sealing ring 134* is secured in an annular grooveformed by opposed circular surfaces formed on the can support plate 123and the encircling clamping ring 132 respectively. The lid supportmember 130 is secured by bolts to an upper end plate 115 of theassociated cell closer 98.

Each cell body portion 94 includes an inverted metal cup portion 138*which is carried in a said aperture 88 formed in the upper annular plate16 of the drum unit 14, and is secured therein by bolts which passthrough an integral flange 140* of the cup portion 138. The cup portionhas also an integral upward tubular extension 142*.

An assembly of electrodes 144* is disposed concentrically within the cupportion 138, and is retained therein solely by a retaining ring 146*which is bolted to the lower end of the cup portion. That assembly ofelectrodes includes a central tubular electrode 148* which is flanged atits upper end, and closed at its lower end by an apertured end cap 150*.The upper end of that electrode is electrically separated from theadjacent parts of the cup portion by a thin annular insulator 152*. Aterminal stalk 154* is screwed into the flange of the central electrode,extends upwardly through a tubular insulator 156* mounted and sealed inthe upper wall of the cup portion, and has secured thereon the cable eyeof a first electrical supply cable 158*.

An apertured tubular insulator 160* lines the upper part of the cupportion 138 and encircles the flange of the central electrode 148.Carried within that tubular insulator and lying adjacent to, butelectrically insulated from, the flange of the central electrode is atubular, can-contacting electrode 162*, which is provided internally atits lower end with a counter-bore 164* for receiving and makingelectrical connection with the outwardly-turned, upper rim 166* of a can128 that is to be electro-coated. A second terminal stalk 168* isscrewed into the upper end of that tubular electrode, extends radiallyoutwards through a tubular insulator 170* mounted and sealed in the sidewall 172* of the cup portion 138, and has secured thereon the cable eyeof a second electrical supply cable 174*.

A tubular external electrode 176* is supported within the cup portion138 by means of an external shoulder 178*, which is carried on theretaining ring 146, and by means of an electrically insulating spacerring 179*. The upper parts of that external electrode adjoin the tubularinsulator 160 and the tubular can-contacting electrode 162. Insulatingwashers 180*, 182* inserted between adjacent parts of the externalelectrode 176 and the adjoining, can-contacting electrode 162 and theretaining ring 146 respectively provide electrical separation of thoseparts.

A third terminal stalk 184* is screwed into the lower part of theexternal electrode 176, extends radially outwards, and receives thereonthe cable eye of a third electrical supply cable 186* and the cable eyeof an interconnecting cable 188* which is connected at its remote end tothe underside of the can support plate 123 by a bolt 190*.

Secured and sealed in the uppermost end of the tubular extension 142 ofthe cup portion 138 is a fluid supply tube 192* which is provided at itslower, end with a normally-closed, non-return rubber valve member 194*,and at its upper end with a supply pipe 196*. That valve member 194permits the flow of fluid from the supply pipe 196 into theelectro-coating cell only when the fluid pressure above it is sufficientto open the valve member; hence, it prevents the loss of electro-coatingfluid from the supply tube 192 into the cell when that fluid pressure iscut off from the supply tube.

The tubular extension 142 of the cup portion 138 has near its upper enda transverse port 198* which communicates via a non-return valve unit200* secured to the tubular extension 142 with a low pressure air supplypipe 202*. That valve unit incorporates a conical rubber valve memberwhich rests on an apertured conical seat, and permits the flow of lowpressure air into the cell body 94 via the annular space surrounding thecentral electrode 148, but prevents the exit of electro-coating fluidfrom the cell body.

The lid support member 130 is provided with a fluid passage 204* forsupplying fluid to a port 206* disposed centrally in the can supportplate 23. At its end remote from the port 206* that passage includes avertical inlet section 208* which terminates at its upper end in afemale frustoconical valve seat 210*.

The lid support member 130 also carries an upright valve-actuating pushrod 212*. Adjacent lid support members are oppositely handed, to permitthe push rods in each pair of adjacent lid support members to lieadjacent one another.

The upper annular plate 16 of the drum unit 14 carries on its uppersurface sixteen twin-valve units 214* (see FIGS. 2 and 5) disposedradially inwards of the respective pairs of cell bodies 94. Each suchvalve unit 214 has an inlet passage 216* connected to one of said fluidfeed pipes 74. That inlet passage is connected through respectivenormally-closed, poppet valves 218*, 220* with respective outletpassages 222*, 224*. Each such outlet passage is connected, on the onehand, upwardly with the said supply pipe 196 of the associated cell body94, and on the other hand, downwardly with a downwardly-pointingnon-return valve unit 226*, 228* situated on the underside of the drumunit upper plate 16. Those non-return valve units incorporate rubbervalve members 229* which are similar in construction and operation tothe valve members 194 which close the lower ends of the cell supplytubes 192, and have downwardly-pointing, resilient, male outlet nozzles230*, 232*, each of which is arranged to engage, when the associated lidis raised to close the cell, in the said female valve seat 210 of theassociated lid support member 130, and so complete a flow passage to thecentral port 206 in the can support plate 123.

The push rods 212 carried by the two lid support members 130 of theassociated cell closers are aligned vertically with the respectivetappets 234*, 236* which depend from the valve unit 214, project throughthe drum unit upper plate 16, and are operable by respective push rods212 whereby to effect operation of the respective poppet valves.

Thus, when a cell closer 98 operates to raise the associated lid supportmember 130 and so close the associated cell 96, closure of the celloccurs simultaneously with the closure of the associated valve outputnozzle (e.g. 230) on to its associated valve seat 210 and with theraising of the associated poppet valve (e.g. 218) to cause the flow offluid from the feed pipe 74 to the associated cell body 94 via the upperfeed pipe 196 whereby to flood the interior of a can 128 present in andto be coated in the cell, and simultaneously to the central inlet port206 formed in the can support plate 123 whereby to rapidly immerse theexterior of that can 128.

Coating fluid admitted to the cell body via the fluid supply pipe 196,tube 192 and the non-return valve member 194 passes into contact withthe interior of the can 128 to be coated via a series of longitudinalpassages 238* formed in the end cap 150, and if necessary, via a seriesof radial passages 240* formed in or adjacent the end cap 150; and afterfilling the can rises to the level of, and escapes from the cell bodyvia, radial apertures 242* formed in the upper parts of thecan-contacting electrode 162 and encircling tubular insulator 160. Thefluid passes thence via an annular gallery 244* formed in the cupportion 138 and two circumferentially-spaced, oblique outlet passages246* to two flexible exhaust pipes 248*. Those pipes pass through theinternal walls 20 and 24 of the drum unit 14 to discharge their flows onto the upper funnel member 50, from where it flows via the collector pan54 and exit holes 60 to the reservoir 62.

Coating fluid admitted to the closed cell via the central inlet aperture206 in the can support plate 123 rises around the can 128 to the levelof the upper rim 166 of the can from where it exits via radial andlongitudinal passages 250* formed in and between the can-contactingelectrode 162 and the encircling tubular insulator 160, to the gallery244.

Referring now to the FIGS. 6 and 7, the slip-ring unit 34 comprisesessentially an outer annular plate 252* having dependent therefromradially-spaced outer, inner and intermediate cylindrical walls 254*,256*, 258*. The intermediate wall 258 is secured on an annulus 260*,which is itself secured on four equi-spaced, hollow, vertical columns262* which are carried by an annulus 263* secured on the innerperipheral part 30 of the drum unit upper plate 16.

The annular plate 252 carries a central bearing support disc 264* fromthe centre of which a hollow bearing shaft 266* rises. A bearing sleeve268* is carried on that bearing shaft 266 by two vertically-spacedbearing races 270*, and carries itself the brush gear unit 38. Thebearing shaft 266 and associated bearing sleeve 268 constitute the saidcentral bearing unit 36.

The outer cylindrical wall 254 carries externally thereon threevertically-spaced circles 272*, 274*, 276* of slip-ring segments 278*,which segments are identical with one another and are electricallyinsulated from one another and from the cylindrical wall carrying them.Permanent electrical connections with those slip-ring segments are madeinternally of the wall 254 by means of electrically insulated connectionstalks 280* which pass through that wall and secure the segments inposition thereon. Each circle of segments comprises thirty-two segments,i.e. one for each of the cells 96.

The slip-ring segments of the upper circle 272 receive on theirrespective connection stalks 280 the remote ends of the respective saidfirst electrical supply cables 158 which are connected to the interiorelectrodes 148 of the respective cells.

The slip-ring segments of the middle circle 274 receive on theirrespective connection stalks 280 the remote ends of the respective saidthird electrical supply cables 186 which are connected to the externalelectrodes 176 and 123 of the respective cells.

The slip-ring segments of the lower circle 276 receive on theirrespective connection stalks 280 the remote ends of the respective saidsecond electrical supply cables 174 which are connected to thecan-contacting electrodes 162 of the respective cells.

Slip-ring segments which are in vertical alignment in the three circlesare associated with the various electrodes of the same cell 96. The saidelectrical supply cables are carried downwards to the respective cellsthrough the respective hollow vertical columns 262.

The brush gear unit 38 comprises a circular brush carrier plate 282*which is carried centrally by the bearing sleeve 268, and which hasdependent therefrom, around a predetermined portion thereof, seventeencircumferentially-spaced brush support posts 284*. Each said post isprovided with an electrically-insulating support member, and carriesthereon, one above the other, three brush boxes 285* in which carbonbrushes 286* are urged by biassing spring means (not shown) into contactwith vertically-aligned slip-ring segments. The angular pitch of thebrush support posts 284 is equal to that of the electro-coating cells96, and hence to that of the slip-ring segments.

Each brush support post 284 has associated therewith three electricalterminal stalks 288*, 290*, 292* which are secured in and extend throughthe brush carrier plate 282 to positions adjacent the respective brushboxes 285, at which positions the flexible connections 287* of therespective brushes 286 are connected to those respective terminalstalks.

Groups of three electric supply cables 294*, 296*, 298* are connected tothe respective groups of terminal stalks 288, 290, 292, and are carriedaway to appropriate electric supply terminals of a d.c. supply source300* which is connected to and controlled by a control and monitoringapparatus 302*. That supply source is fed from an a.c. supply system(not shown), and incorporates a full-wave, thyristor bridge rectifiercircuit to deliver the requisite d.c. voltage.

The slip-ring unit 34 includes a lower, horizontal cover plate 304*which extends radially outwards from the annulus 260 to meet in spacedrelationship an outer, vertical cover plate 306* which is carriedperipherally by the brush carrier plate 282.

The low pressure air supply pipes 202 of the respective cells 96 havetheir upper ends connected to pipes 308* which are dependent from theslip-ring carrier 252, 264 and which open at their respective upper endsinto ports 310* formed around the upper peripheral plane surface 312* ofthe bearing plate 264.

A kidney-shaped manifold 314* lies on that plane annular surface 312,covers a predetermined number of the ports 310 in that surface, and isrestrained against circumferential movement by upright posts 316* whichare secured to the top of the manifold 314 and which slidably extendthrough a cover plate 318* which covers a kidney-shaped access openingformed in the brush carrier plate 282. The manifold is urged into closesliding contact with that plane annular surface 312 by compressionsprings 320* carried on the location posts 316 and trapped under thecover plate.

A low pressure, high flow rate air supply source (not shown) isconnectable with the manifold 314 by means of a connector 324* which issecured in the cover plate 318 and which extends in fluid-tight slidingrelation through the top of the manifold 314.

Mounted at the front of the apparatus so far described are infeed andoutfeed devices 326*, 328* which are driven by the rotatable drum unit14 by means of gear wheels 330*, 332* which mesh with the toothed gearring 44 of that unit.

THE INFEED DEVICE

Referring now to the FIGS. 8 to 10, the infeed device 326 includes arotatable turret 334* carrying a series of eight can holders or pockets336* which are spaced apart around the turret and which are arranged totransfer, on rotation of the drum unit, respective cans 128 supplied toit by a synchronised screw feed-conveyor 338* to respective cell closers98 as they pass through a predetermined first or infeed station adjacentthe infeed device. A guide rail 340* extending part way round the turretcauses the cans to follow, as they are urged along by one of said canholders, a predetermined arcuate path from the conveyor 338 to the cellcloser then at the infeed station.

As shown in the FIG. 10, each can holder 336 is carried within theturret 334 on a retractable shaft 342* which is biassed by an helicalcompression spring 344* to an outer can-guiding position. That shafthas, within the turret, a cam follower wheel 346* which is urged by theaction of that biassing spring radially outwards against the shapedinternal surface of a static, generally circular cam 348*. That camsurface is shaped so as when the can holder moves into the position fordepositing a can on the cell lid then at the infeed station, it isgradually and temporarily retracted slightly so as to enable the canholder to follow for a short way the locus 350* of the cell lid 100 asit moves into, through and beyond the infeed station. This enables thecan to be properly transferred to and positioned on the cell lid, sincethe can holder and cell lid move for a short way along the same locus.The guide rail 340 is also shaped as shown at 351* so as to cause thecan to move into and along that locus 350.

In more detail, the infeed turret 334 comprises an inverted cup-shapedmember 500* secured at the top of a driving shaft 502* which rises,through a tubular shroud 504*, from a torque-limiting device (not shown)which is connected to the gear wheel 330. That shroud passes upwardlythrough a transverse support channel 506* which carries on its surface afixed, turret-location socket 508*. Secured in that socket is a turretsupport means 510* which incorporates a lubricating oil reservoir tank512*. An upstanding tubular member 514* constituting an inner wall ofthat oil tank encloses the driving shaft 502 which extends therethroughwith clearance. A seal 516* provided at the upper end of that tubularmember 514 prevents the exit of lubricating oil between that member andthe driving shaft.

The turret incorporates a dependent, inner cylindrical wall 518* ofwhich the lower end is rotatably supported on a ball bearing race 520*which is itself carried on a transverse platform 522*. That platform issecured by screws on an upstanding intermediate cylindrical wall 524*which rises out of the oil tank and which has in its lower part anaperture 526* to allow circulation of the lubricating oil within thetank.

An oil pumping sleeve 528* encircles the upstanding tubular member 514,is secured at its upper end in the inner dependent wall 518 of theturret, and has formed in its bore a spiral, oil-pumping groove 530*.Thus, on rotation of the turret oil from the reservoir rises up thespiral groove and is delivered at the upper end of the sleeve 528 into aplurality of radial, distribution ducts 532* from where it escapes viavertical nozzles 534* into the spaces enclosed below.

The lower rim of the outer cylindrical wall 536* of the turret has anannular groove 538* into which extends the thin upper rim of the outercylindrical wall 540* of the oil reservoir tank 512, in such manner asto prevent the ingress of electro-coating fluid into the reservoir tank.The lubricating oil surface level indicated at 542* is maintained at aheight such as to prevent the loss of oil between the tongue-and-groovefunction of the outer walls of the turret and oil tank.

The turret has secured around its circumference at each of eightequi-spaced positions a respective can holder unit 544*, which isremovably carried within radially-aligned, large and small apertures546*, 548* formed respectively in the outer and inner cylindrical walls536 and 518 of the turret.

Each can holder unit comprises a slotted body 550* which is provided atone end with a fixing flange 552* and an adjacent spigot portion 554*for locating and securing (by screws not shown) the unit in position inthe aperture 546, and at the other end with a plug portion 556* whichlocates in said smaller aperture 548.

The retractable shaft 342 supports at its outer end the associated canholder 336, is slidably carried in respective radially-aligned bores558*, 560* formed in the respective end portions of the slotted body550, and has a central waisted portion 562* in which is carried avertical stub shaft 564*. That stub shaft is secured in position by anut 566* which engages the slotted body via a slide block 567* carriedin a slide way formed in the slotted body, and carries rotatably mountedat its lower end the said cam follower wheel 346. The biassing spring344 is trapped on the retractable shaft 342 between shoulders formed onthat shaft and in the slotted body.

The transverse platform 522 has at its outer periphery an upstandingwall 568*, of which the inwardly facing surface constitutes the saidcircular cam 348.

Sealing rings 570* are provided on the retractable shaft, and behind thefixing flange 552, to prevent the ingress of electro-coating fluid intothe turret, and also to prevent the egress of lubrication oil. Annularshrouds in the form of flexible bellows 572* are provided on theretractable shaft 342 and the fixing flange 552, and a tubular extension574* is provided on that flange, all for that same purpose.

THE OUTFEED DEVICE

Referring now to the FIGS. 9 and 11, the outfeed device 328 likewiseincludes a rotatable turret 352*, which turret carries a series of eightcan-grippers 354* spaced apart around it and arranged to receive in turnsuccessive cans that are brought by rotation of the drum unit 14 to apredetermined outfeed station adjacent the outfeed device. Each of thecan-grippers is arranged so as in turn, to lightly grip and remove fromthe cell lid 100 passing through the outfeed station a can presented toit at that station, then as the turret moves on, to rotate the canclockwise (as seen from the turret) about its transverse axis through anangle of 180° so as to empty the coating fluid still remaining in thecan into a trough below (not shown) and thereafter release and depositthe can, open end downwards, on to an outfeed conveyor 356*, and finallyon continued rotation of the turret, to reverse the gripper to itsformer position ready to receive the next can presented to it at theoutfeed station.

Each can gripper 354 is carried within the turret 352 on a rotatableshaft 358* having gear teeth which engage with those of avertically-reciprocable gear rack 360*. That rack is spring biassed by acompression spring 361* to its lowermost position, and has associatedwith it, within the turret, a cam follower wheel 362* which cooperateswith a static annular cam 364* of cyclically varying height. When thegripper is at the outfeed station ready to pick up a can, the height ofthe cam beneath the follower wheel is at a maximum value.

During rotation of the turret through a first half revolution from theoutfeed station, the cam allows the rack to move temporarily to a lowerposition and then, during the second half revolution, causes the rack toreturn to its biassed upper position. Such movement of the rack thusrotates the associated gripper shaft 358 through 180° (thereby rotatinga gripped can so as to empty its contents in the direction of rotationof the turret), and then returns it to its former position, all withinthe course of one complete revolution of the turret, so as to achievethe desired can gripper operation.

Each can gripper 354 includes a movable jaw member 366* which is biassedto a closed can-gripping position, and which is operated within theturret by a cam follower 368* which is biassed radially inwards intocontact with a static cam 370* of cyclically varying radius. That camand follower arrangement is arranged so as (a) to present the gripper inits open, can-receiving condition to the can then moving into theoutfeed station, (b) then as the turret rotates to move the gripperthrough that station, to allow the movable jaw member to close lightlyand temporarily on to the can so as to grip it during the followingperiod whilst it is being rotated to the mouth-down position, andfinally (c) to return the jaw member to its open position as the gripperapproaches the outfeed conveyor, so as to release the gripped can on tothat conveyor. The gripper jaw member remains open until after thegripper has been carried round into engagement with the next can to begripped and conveyed by that gripper.

In more detail, the outfeed turret 352 includes, beneath a protectivecan 600*, a drum 602* which comprises a generally cylindrical outer wall604* supported by two vertically spaced, integral, transverse walls606*, 608*. The drum is rotatably carried by complementary taper bearingraces 610*, 612* which engage externally with said transverse walls 606,608, and internally with an upstanding tubular bearing member 614*. Thatbearing member extends upwardly from the inner wall 616* of an integralannular oil reservoir tank 618*, which tank has an upstanding outercylindrical wall 620*. That wall has its upper rim extending upwardlyinto a groove formed in the lower rim of the cylindrical drum wall, in amanner such as to exclude electro-coating fluid from the turret.

An annular turret location plate 622* screwed to the base wall 624* ofthe reservoir tank 618 has a lower, spigot portion which engages in aturret locating socket 626* which is itself secured on a transversechannel 628*.

An oil pumping sleeve 630* lines the tubular bearing member 614, issupported at its base by a ball bearing race 632* which is trapped in arecess in the oil tank base wall 624 by the turret location plate 622,and is provided in its external cylindrical surface with a spiral oilpumping groove 634*.

A turret driving shaft 636* rises from a torque-limiting device (notshown) which is connected to the gear wheel 332, through a tubularshroud 638*, the support channel 628, the location socket 626, and theoil pumping sleeve 630, and is secured by adjustable coupling means 640*to a transverse circular driving plate 642* which is secured to theupper drum wall 606 by screws 644*. Axially extending teeth 646* formedat the upper end of the oil pumping sleeve 630 engage in driving slotsformed internally in the driving plate 642.

Lubricating oil pumped to the top of the pumping sleeve 630 flowsdownwardly (a) over a baffle plate 648* in which are provided verticaloil holes for directing oil into the upper bearing race 610, and (b)outwardly through transverse radial passages 650* to lubricate the othermoving parts that are enclosed within the turret.

The turret has secured around its circumference at each of eightequi-spaced positions a respective can gripper unit 652*, which isremovably carried in an aperture 654* formed in the cylindrical wall604. Each can gripper unit 652 comprises a flanged body 656* secured insaid aperture 654, by screws 657*, and having an annular closure member658* secured thereto, by screws 660*. That closure member secures inposition a ball bearing race 662* in which said rotatable shaft 358 isjournalled for rotation. That shaft comprises an assembly consisting of(a) a pinion 664* and an integral shaft 666* which is received in saidball bearing race 662, and (c) a gripper support member 670* whichprotrudes through said closure member 658, all such parts being securedfor rotation together.

Sealing rings 672* are provided on either side of the ball bearing raceso as to exclude therefrom lubricating oil from within the turret andelectro-coating fluid from outside the turret. The closure member 658and gripper support bush 668 also carry baffles 674* for minimising thepenetration of such fluid.

The flanged body 656 has adjacent the pinion 664 an opening throughwhich the associated vertically reciprocable gear rack 360 extends andmeshes with said pinion. That rack has upper and lower support shafts676*, 678* which are slidably carried via bearing bushes 680*, 682* inthe upper and lower transverse walls 606, 608 of the turret drum. Theupper support shaft 676 carries around it the said compression spring361, whilst the lower support shaft carries at its lower end, on atransverse pin 684*, a ball bearing race 686*, of which the outer racemember constitutes the said cam follower wheel 362.

An annular cam unit 688* is secured on the base wall 624 of the oil tank618, and has an upstanding cylindrical wall 690* of varying height,which wall is positioned beneath and supports the said cam followerwheel 362, and so constitutes the said annular cam 364.

The gear rack and associated parts are lubricated by oil dropping fromthe radial passageways 650.

The rotatable shaft assembly 358 has a central bore in which axiallyspaced bearing surfaces 692*, 694* formed in the pinion shaft 666 carrya slidable gripper operating shaft 696*. That gripper operating shaftcarries (a) at its inner end, said cam follower 368 which is constitutedby a ball bearing 698* rotatably held in a bearing socket 700*, (b) atits outer end, a gripper operating button 702* which protrudes beyondthe extremity of the gripper support member 670, and (c) intermediateits ends, a compression spring 704* trapped between opposed shouldersformed on the shaft 696 and in the bore of the pinion shaft 666, forbiassing the gripper operating shaft radially inwards of the turret.

The cam follower ball 698 rests in contact with the outer surface of acam ring 706* which encircles and is secured by screws to the centraltubular bearing member 614. That cam ring has varying radial depth, andconstitutes the said static cam 370 for operating the associatedgripper, via said gripper operating shaft 696.

The construction of one said can gripper 354 is best seen in the FIG.12, where it is shown detached from the turret. The can grippercomprises a gripper block 708* having formed in its rear face acylindrical mounting socket 710* arranged for engagement on a plugportion 711* formed on said gripper support member 670. The gripperblock is arranged to be secured on that support member 670 by threescrews 712* which are sunk in respective counter-bored holes formed inthe gripper block.

The front face of the gripper block is symmetrically shaped at 714* tosuit the cylindrical shape of a said can 128 that is to be transportedand emptied by the gripper, and that face is relieved at spacedvertically-extending regions 716*, to leave fourcircumferentially-spaced can-containing surfaces 718*.

The gripper block is sandwiched between two jaw plates 720*, 722*, whichare spaced apart and from the gripper block by four spacer pins724*-730*. Countersunk fixing screws 732* received in the respectiveends of those spacer pins pass through and so clamp the jaw plates tothe spacer pins so as to form a said gripper jaw member 366. The threepins 724 to 728 are similar and constitute simple, butted spacer pinsfor securing together the gripper plates at the desired spacing. Thepins 724, 726, and 730 pass with substantial clearance through holes734* formed in the gripper block. The pin 730 has (a) end portions ofreduced diameter which engage in recesses formed in the jaw plates andcarry spacer rings 736*, and (b) a central bearing portion 738* which isjournalled in a bearing hole 740* formed in the gripper block. Hence,the jaw member 366 is pivotally mounted on the gripper block by means ofthe spacer pin 730. Sealing rings 742* encircle the spacer rings 736 andserve to exclude electro-coating fluid from the cooperating bearingsurfaces of the gripper block and jaw member.

The gripper block is provided with a first screwed bore 744* whichintersects with the clearance hole 734 that houses the spacer pin 724. Abias compression spring 746* is trapped in that bore and is urged intocontact with that spacer pin 724 by a grub screw 748*. The gripper blockis also provided with a second screwed bore 750* aligned with said firstscrewed bore 744 and in which is screwed an adjustment stud 752* forsetting a biased, "closed" position of the jaw member 366 relative tothe gripper block 708.

The gripper block is also provided with a bore 754*, and a counter-bore756*, having an axis which intersects with that of the clearance hole734 housing the spacer pin 726. That counter-bore constitutes theaforesaid socket 710 for receiving the plug portion 711 of the rotatablegripper support member 670.

When a gripper assembly 354 is mounted and secured on a gripper supportmember 670, the gripper operating button 702 rests adjacent but nottouching the jaw operating spacer pin 726, so that the jaw member isbiased to the closed position dictated by the setting of the adjustmentstud 752. On rotation of the gripper turret, the static cam ring 706cyclically and temporarily presses the cam follower 698, 700 and gripperoperating shaft 696 radially outwards against the thrust of the biassingspring 698, thus causing the jaw operating button 702 to press againstand temporarily displace the spacer pin 726 and so temporarily open thegripper jaw member relative to the gripper block.

The jaw plates 720, 722 are shaped in the manner shown, and have each acan-gripping land 758* spaced from a can-ejecting land 760* by arelieved region 762*. Those can-contacting lands are positioned inrelation to the can-contacting lands 718 of the gripper block such thatwhen the jaw member is in the closed position gripping a can, that canis contacted by those lands over a circumferential length which exceedsby a small amount half the circumference of the can.

The entrance to the space enclosed by the gripper block 708 and jawmember 366 is inclined relative to the axis of rotation of the gripperat an angle of approximately 25°, which angle is dependent on therelative diameters of the two circular paths followed by a can whentravelling respectively (a) on a cell lid 100, and (b) in the grip of agripper, and is determined to suit the path relative to a gripper of acan entering the gripper at the outfeed station.

For a can of a given diameter, that entrance to the space enclosed bythe gripper, when the jaw member 366 is in the open position, has adimension approximately 1 mm greater than the can diameter. A movementof approximately 1 mm of the can-contacting land 758 of the jaw member366 between the open and closed positions suffices to enablesatisfactory gripping and releasing of those cans.

That small movement of the jaw member is possible since the locus of thecan relative to the gripper when travelling from a cell lid into theopen gripper is substantially the same as that when travelling from thegripper on to the outfeed conveyor 356, the gripper having inverteditself and the turret rotation having reversed the direction of travelof the can between the moments of gripping the can and subsequentlyreleasing it.

The closed position of the jaw member is adjusted so that the pinchexerted on the electro-coated cans is minimal, and such that no damageis done to the coating newly applied to those cans when the cans arecontacted by the said can-contacting lands of the gripper block and jawmember.

On actuation of the gripper operating shaft 696 at a time for openingthe gripper, the consequent opening movement of the gripper jaw member366 relative to the gripper block 708 results in the application of acan-ejecting pressure on the can by the can-ejecting lands 760, so thatthe can is then moved positively out of contact with the can-contactinglands 718 and falls freely on to the outfeed conveyor. This ensures aprompt release of the can at the time for depositing the can on to thatconveyor.

The mode of operation of the apparatus so far described will now bedescribed with reference to the Figures already described above and tothe FIGS. 15 and 19.

FIG. 15 shows in relation to a diagrammatic plan view of the rotatableturntable 14 and its electro-coating cells 96, various events that occurduring the rotation of the turntable through one revolution, and thatcycle of events will now be described below.

FIG. 19 shows schematically in relation to a similar diagrammatic planview of the turntable the various electrical supply, monitoring andcontrol means and activities that constitute the aforementioned supplyand control apparatus 300 and 302.

In operation, the turntable drum 14 and its associated infeed andoutfeed devices 326, 328 rotate in synchronism at a constant speeddetermined by that of the can production/processing line of which theelectro-coating apparatus forms part; the pump 66 provides a supply ofelectro-coating fluid under pressure to the cell supply valve units 214via the central pipe 68, the associated distribution chamber 70, 72 andthe distribution pipes 74; high pressure air is supplied to theinterconnected cell closure cylinders 102 via the rotating supplycoupling 106, the manifold 108 and the feed pipes 110, so that all ofthe respective cell lids 100 are urged upwardly towards their upperpositions; low pressure air is supplied to the kidney-shaped manifold314, and thence to the respective supply pipes 202 that are temporarilyconnected therewith, the associated non-return valves 200 and the cellbodies 94; and the electrical supply source 300 and associated controlapparatus 302 for energising the respective brush sets are renderedoperative.

Cans are delivered upright, i.e. with base wall lowermost, via acontrollable "can-stop" device 400* (see FIG. 19) (for stopping the flowof cans when necessary) to the screw-feed conveyor 338 which spaces thecans apart and delivers them at appropriate intervals to the infeeddevice 326.

Each can is guided by a can holder (or pocket) 336 of the infeed devicein succession past (a) a "can-in" proximity sensor 402* whose functionis to signal to the control apparatus 302 the presence of a can 128 inthe pocket 336 passing the sensor, and (b) a "pre-weighed can" sensor404* for sensing marks placed on specific cans which have been weighedbefore being introduced into the flow of cans, and for signalling to thecontorl apparatus 302 the passage thereby of each such marked can.

On further rotation of the infeed device and turntable, each can isdelivered in turn by the associated can pocket 336 to the lowered celllid 100 then arriving at the infeed station, and is deposited there onto the support pins 126 which project from the can support plate 123.That cell lid is held temporarily in an appropriate lowered position bythe static cam 122.

Since each can that is delivered to the infeed station undergoes thesame procedure, the progress of one can only will be followed through atypical operating cycle of the turntable 14.

During rotation of the drum unit 14 through one revolution, each of theelectro-coating cells 96 and its associated parts are carried roundthrough thirty-two successive, equi-spaced positions or zones relativeto the base structure 10. Those positions will be referred to in whatfollows as "station 1", "stations 2", etc and as a datum, station 1 willbe taken to be the infeed station at which a can to be electro-coated isintroduced for enclosure in an electro-coating cell.

During the passage of the cell lid on which the can has been dsepositedthrough the next three stations at the height of the static cam 120progressively reduces, thus allowing the associated cam follower 118 torise under the pressure of the compressed air supplied to the associatedcell closer cylinder 102, and the cell lid to close and seal theassociated cell body, thus totally enclosing the can, making electricalcontact with it through the can-contacting electrode 162, and holding itfirmly on the support pins 126.

A "cell-closed" proximity sensor 406* disposed adjacent the static cam122 is arranged to detect the presence of each cell closer cam followerwheel 118 at its highest, "cell-closed" position as it passes thereby,and in response to such a presence to supply to said control apparatus302 a signal indicating that the passing cell is properly closed andready to receive electro-coating fluid.

The final upward movement of the cell closer also causes the associatedpush rod 212 to operate the associated poppet valve 218, 220 of the cellsupply valve unit 214 and so permit electro-coating fluid to flowrapidly to the cell body 94 via the supply pipe 196, and simultaneouslyto the can support plate 123 via the non-return valve unit 226, 228thereby at the same time to completely fill and immerse the can 128 witha rapidly flowing electro-coating fluid.

The fluid continuously leaves the cell, after flowing in contact withone of the surfaces of the can, by way of the exhaust ports 246 andpipes 248, and returns to the reservoir tank 62 for recirculation by thepump 66. At station "5" and a predetermined small group of laterstations, the flow of fluid to the cell is at a maximum rate, since theflow to the supply pipe 74 is unimpeded by the larger ports 80 in thecylindrical wall 78 of the baffle 76. The fully immersed condition ofthe can is reached only after the cell has moved on to another, laterstation, e.g. station "8".

The flow path of the electro-coating fluid after entering the central,vertical supply tube 68 is shown in the FIG. 13, where all of the partsthat enclose the flow path are indicated with the same form ofcross-hatching, for the sake of simplicity. Likewise, the flow paththrough an electro-coating cell 96 is shown in greater detail in theFIG. 14, but in this case the various components through which the fluidflows are cross-hatched in appropriately different manners.

When the cell reaches, for example, the station 10, the control andmonitoring apparatus 302 associated with the electrical supply source300 is effective to apply small test voltages between the enclosed can128 and the internal and external electrodes 148, 176, 123 via theassociated slip-ring segments and the brushes associated with thatstation, whereby to carry out a short circuit test (for instance byobserving a loss of charge in a precharged cell, or by measuring thecircuit resistance between the can and the internal and externalelectrodes), and to determine from the response thereto whether or not ashort circuit exists between the can 128 and either of the internal andexternal electrodes 148, 176, 123. After completing that test, andsignalling to the control apparatus 302 that there is no short circuitpresent in the closed cell, the electro-coating process can commence,provided that the control apparatus 302 has already received in respectof that cell the other necessary feedback signals indicating that (a) acan is present in the cell, and (b) the cell is properly closed.

At that time, the rate of flow of the fluid through the cell isgradually reduced to a lower value, and remains thereafter at that lowervalue, by reason of the juxtaposition of a smaller, flow-restrictingport 80 in the baffle wall 78 with the port leading to associated cellsupply pipe 74.

During the progression of the cell through each in turn of apredetermined group of the later stations, the power supply source 300(which includes an ON/OFF controlled thyristor bridge rectifier circuit)applies a predetermined d.c. voltage pulse across the requisite brushesof the vertical set associated with the particular station, and henceacross the relevant slip-ring segments associated with the cell, so asto pass a direct current pulse between the internal electrode 148 andthe can 128, and so cause coating material to be electro-phoreticallydeposited from the fluid on to the internal surface of the can.

Each such pulse is initiated only when full contact is made betweenthose slip-ring segments and the whole contact areas of the energisedbrushes, and is terminated just before those slip-ring segments breaktheir full contact with the whole contact areas of those brushes. Thisensures that electro-coating current flows for the maximum possibletime, and is interrupted for the minimum time interval. For reasons thatwill be explained later, this is highly beneficial. Moreover, thisavoids the possibility of drawings sparks and ares between the energisedbrushes and the slip-ring segments on making and breaking electricalcontact therebetween. Control of the duration of the voltage pulse maybe effected by either a timing circuit synchronised with the turntablerotation, or by a turntable position responsive circuit, the latterbeing preferred.

Referring now to the schematic diagram shown in the FIG. 16, theelectrical control and monitoring apparatus 302 associated with thesupply source 300 includes (a) integrating means 372* for summating thequantity of electrical charge (Coulombs) delivered to the cell duringthe passage of each such current pulse, (b) summating means 373* forsummating at the end of each such pulse the total number of Coulombsdelivered so far to the cell in all of the respective pulses; (c)comparison means 374* for comparing with a preset reference value thattotal charge; and (d) means 376* for inhibiting the delivery of furthercurrent pulses to the cell during its further progress through theremaining stations whenever the total number of Coulombs delivered sofar exceeds that preset value. By this means, the deposit of therequired thickness (or weight) of coating on the interior surface of thecan is achieved safely and efficiently, and in the minimum of time, aswill be explained later.

The electrical control and monitoring apparatus 302 also includes a cellprotection means 378* for sensing from the current and voltage deliveredto the cell the onset of a short circuit condition in the cell whilstcurrent is being delivered thereto, and for providing in response tosuch a sensed condition an output signal for (a) suppressing as rapidlyas possible the voltage supplied by the supply source 300 to the cell,and (b) closing without any undue delay a low resistance divertercircuit 380* (referred to later for convenience as a "crowbar" circuit)which is connected directly across the output circuit of the thyristorbridge circuit which supplies the cell circuit. The prompt closure ofthat diverter circuit before the supply source voltage dies away rapidlyreduces to a low value the voltage developed across and hence thecurrent flowing in the cell, so that the risk of causing damage to thecell is minimised.

For determining when the associated vertical set of slip-ring segmentsis fully in contact with, and is subsequently about to break fullcontact with, the requisite vertical set of brushes at a particularstation, the peripheral part of the annular plate 252 of the slip-ringunit 34 has formed therein a series of equi-spaced, semi-circularnotches 382* which correspond with the respective slip-ring segments.Two static proximity detectors 386*, 388* are mounted on the brushcarrier plate 282 adjacent the notched peripheral part of the plate 252,and are spaced apart by a dimension which is determined by the sum of(a) the width of a brush 286, (b) the width of the air gaps 389* betweenadjacent slip-ring segments, and (c) the maximum segment travel that canoccur during the switching-off response time. The proximity detectors386, 388 detect in succession the passage thereby of each notch 382, andsupply in response thereto "switch-on" and "switch-off" signals to thecontrol and monitoring apparatus 302 so as to indicate the passage ofthose notches, and hence of the slip-ring air gaps, relative to theleading and trailing edges of the respective vertical sets of brushes286.

A third static proximity detector 390* is mounted in the brush carrierplate 282 and is arranged to detect the passage of a datum marker 392*which is secured on the top of the slip-ring carrier plate 252. Thatdetector provides for the control and monitoring apparatus 302 aturntable "zero" or datum signal which in conjunction with the signalsprovided by the other two detectors 386, 388 enables the control andmonitoring apparatus (a) to correctly initiate and terminate theelectro-coating current pulses to be supplied to each particular cellduring its progression through the respective stations, and (b) toperform its other functions at the various other stations.

If the external surface, as well as the internal surface, of the can isto be electro-coated during the process, the application of thenecessary current pulses to the brushes which contact the slip-ringsegments of the middle circle 274 is delayed until the cell has movedinto, for example, the station 17. The amounts of charge delivered tothe external electrodes 176 and 123 are likewise measured and summatedat the end of each successive pulse to determine the total charge thathas been supplied so far for electro-coating the external surface of thecan. Likewise, the application of any further current pulses to theexternal electrodes 176, 122 is inhibited when the total charge alreadysupplied at the end of the last pulse exceeds a preset reference valueappropriate to the desired thickness and weight of the external coatingto be applied.

It will be appreciated that since the magnitude of the successivecurrent pulses supplied to any particular cell decays as the coatingprocess proceeds, initially at a relatively high rate, and then at aprogressively decreasing rate, the delaying of the current pulses forproviding the external coating until later in the process of providingthe internal coating has the effect of reducing the maximum currentsupplied to the can-contacting electrode 162, and hence of reducing thesizes of the electrical cables and brushgear supplying it. Controlapparatus similar to the apparatus 372 to 376 are provided for providingthe delayed current pulses for coating the external surface of the cans.

The coating process may proceed, if required, until the cell arrives atthe 25th station. As the cell moves from that station to the next, thestatic cam 120 starts to lower the cam follower 118 (in opposition tothe bias force of the cell closer) and hence the cell closer piston rod114 and its associated cell lid 100 and can 128. The initial downwardmovement of the cell lid cracks open the cell and allows the escape offluid from around the outside of the can into a trough formed around thedrum unit 14 by the intermediate plate 82 and the inner and outer walls20, 84.

That movement also withdraws the associated push rod 212 away from thetappet (e.g. 234) of the cell supply valve 214 and so causes theassociated poppet valve (e.g. 218) to close and so cut off the supply ofelectro-coating fluid to the cell. Simultaneously, the associated lowpressure air supply port 310 in the brush carrier plate 264 moves underand progresses along the kidney-shaped manifold 314, and thereby allowslow pressure air from the manifold to be admitted to the top of the cellbody via the supply pipe 202 and the non-return valve 200 during passageof the cell through this and a small group of other stations that leadup to the outfeed station 29.

This low pressure air supply assists in the rapid reduction of theamount of fluid contained in the can, and moreover, provides an airstabilising force for the can sufficient to maintain it stably inposition on and in contact with the pins 126 of the cell lid 100 as thecell lid descends to its lower position in readiness for removal of thecan.

At the 29th station, the can is engaged gently by a gripper 354 of theoutfeed device 328, is removed from the cell lid, and whilst beingcarried around (by rotation of the outfeed turret 352) by the gripper isrotated through 180° about its transverse axis to empty forwardly thefluid still remaining therein, before being deposited, mouth down, on tothe outfeed conveyor 356.

The control and monitoring apparatus 302 is also arranged to make acomparison, after the cell has passed through the final electro-coatingstation, of the total number of Coulombs supplied to the cell by all ofthe current pulses delivered during the passage of the cell through therespective electro-coating stations, with a preset reference value, andto provide in the event that the total number of Coulombs does notexceed the reference value a "reject" signal signifying that the can hasless than the desired thickness of coating. Such reject signals are usedto activate a reject device 408* positioned alongside the outfeedconveyor 356, and so cause it to direct a blast of air at the passingreject can whereby to sweep it off the outfeed conveyor into a rejectbin.

The marked pre-weighed cans are likewise swept off the outfeed conveyorat a sample retrieval station by a similarly directed blast of airemitted by a sample retrieval device 410* which is activated by thecontrol apparatus 302 at each instant such a marked can passes thereby.

A further proximity sensor 412* ("can-in") is disposed adjacent theoutfeed device 328 at a position just downstream from the delivery pointat which cans are deposited by the outfeed device 328 on to the outfeedconveyor 356. That sensor supplies to the control apparatus 302 a signalwhenever after a can fails to be deposited on to that conveyor, that canstill being carried in a gripper of the outfeed device passes thatsensor. Such "can-in" signals are used in the control apparatus toinitiate immediately an arrest of the turntable 14 and an interruptionof all current flow in the respective cells 96.

An "integrity" check may be carried out (e.g. by an electrical testingmeans) when each can has passed through all of the electro-coatingstations, so as to check the integrity of the deposited coating(s), andto supply to the control apparatus 302 a "fail" signal whenever a canfails that test. Such a signal would give rise to a "reject" signalbeing delivered to the reject device whereby cause the ejection of thefailed can from the outfeed conveyor at the reject station.

The control apparatus 302 includes various shift register means whichare executed in advantageous combinations of hardware and softwaredevices, and which are indicated in the FIG. 19 by the references 414*,416* and 418*. Such register means chart the progress of each cell andthe condition of a can enclosed therein as the cell is carried alongfrom a position upstream of the can-stop device to a position downstreamof the sample station.

The register 414 charts the presence/absence of a can in each cell asthe turntable rotates the cells through the respective stations. Theregister 416 charts for each cell the total charge (in digital form) sofar received by the cell in coating the internal surface of the enclosedcan. The register 418 similarly charts for each cell the total charge(in digital form) so far received by the cell in coating the externalsurface of the enclosed can. The respective shift registers are shown aseach having sixty-four stages, and as receiving shift pulses from thetwo proximity sensors 386 and 388 ("cell on" and "cell off"). One suchsensor provides control signals which initiate the supply ofelectro-coating currents to the cells as the respective sets of brushesmake full contact with the slip-ring segments just moving into contactwith them, and the other such sensor provides control signals whichinitiate the subsequent interruption of those currents just before therespective sets of brushes break their full contact with the slip-ringsegments just moving out of contact with them.

For operating the electro-coating apparatus under a different mode ofcontrol, the control apparatus also includes two additional data shiftregisters 420*, 422* which are similar to the registers 416 and 418respectively. The register 420 receives from means not shown signalsrepresentative of the time periods during which each particular cell hasreceived current in its passage through the respective electro-coatingstations for coating the internal surfaces of the cans. That registerthus charts in its successive stages the respective totals of the timeperiods so far elapsed during which each of the respective cells hasreceived current for coating the internal can surfaces. The register 422similarly receives from means not shown signals representative of thetime periods during which each particular cell has received current inits passage through the respective electro-coating stations for coatingthe external surfaces of the cans. That register thus charts in itssuccessive stages the respective totals of the time periods so farelapsed during which each of the respective cells has received currentfor coating the external can surfaces.

Thus, the determination of when to cease supplying electro-coatingcurrent to each of the respective cells may be made alternatively on thebasis of a comparison with (a) a preset reference value which isindicative of the desired value of total charge to be delivered to eachcell, or (b) an alternative preset reference value which is indicativeof the desired total elapsed time during which current is to flow ineach cell. In the former case the data stored in the two registers 416and 418 is compared with the appropriate reference value (for internalor external coating) of total charge to be delivered, whilst in thelatter case the data stored in the two registers 420 and 422 is comparedwith the appropriate reference value (for internal or external coating)of the elapsed time during which current is to flow in each cell.

In this latter case (total elapsed time basis), the apparatus operatesto ensure that each cell receives current (at a level which is intendedto deliver the desired coating during the reference value of elapsedtime), and to reject any coated can that in a comparison carried out atthe end of the electro-coating process with a further reference value(indicative of the desired Coulomb count for achieving the desiredcoating) is found to have received less than the Coulomb count necessaryfor achieving the desired weight of deposited coating material.

If desired, the low pressure air supply may be connected via themanifold 314 and the pipe 202 with a cell 96 during its passage throughone or more stations immediately after a can has been deposited on itscell lid 100 at the infeed station, for the purpose of stabilising thatcan in position on the cell lid.

Logging in a micro-processor the various reject signals, together withthe identities of the cells in which the rejected cans were coated,enables the operator of the apparatus to determine which particularcells may be in need of attention or replacement.

The provision of the two non-return fluid valves 194 and 226/228 in therespective fluid paths feeding the interior and the exterior of the canprovides the advantage that when the cell is opened at the end of theelectro-coating process a large volume of the electro-coating fluid istrapped between those two valves and is thus conserved for use duringthe next electro-coating cycle. This has a considerable and beneficialinfluence on the capacity and rating of the fluid circulation pump 66,and moreover, reduces the time required to fill the cell.

The interconnection of the cylinders 102 of all of the cell closers 98in a ring system enables the demand for high pressure air to beminimised, since air being expelled from cylinders as they approach theoutfeed station is taken up by cylinders just leaving the infeedstation.

ELECTRICAL CIRCUIT AND CONTROL APPARATUS

An electrical circuit and control apparatus for giving effect to theabove described mode of operation will now be described, after a briefdiscussion of a prior art control technique and circuit of which thepresent invention is an improvement.

In our patent specification GB No. 2,285,922 B (to which the reader'sattention is hereby directed for further information concerning itsdisclosure), we disclosed an electro-coating apparatus in which canbodies were successively enclosed in respective cells carried incircular formation around a rotating turntable. The electrode systems ofthose cells were electrically energised successively as they werecarried round by the turntable through successive electro-coatingstations, so as to cause three successive depositions of electro-coatingmaterial on to the interior surface of each can body from anelectro-coating fluid flowing in contact with that surface.

At each of those successive stations, a stationary pair of brushesengaged successive pairs of slip-ring segments carried by the turntableas they passed by on rotation of the turntable, thus energisingsuccessively the cell electrode systems that were electricallyassociated with the respective pairs of slip-ring segments, each cellhaving its electrode system connected in series between the respectivesegments of one of said segment pair.

The application of a common d.c. voltage across the three sets ofbrushes thus resulted in three successive energisations of each cellelectrode system. Electronic switches placed in series with therespective circuits supplying the repective brush pairs enabledindependent switching on or off of the voltage applied to the respectivebrush pairs, and thus independent control of the electro-coating processin the respective stations and in the respective cells.

Synchronising means ensured that the application of voltage to therespective brush pairs occurred only after the respective brush pairswere fully in contact with the respective segment pairs which had justmoved into electrical contact with them.

The timing control means also ensured that the voltage applied to therespective brush pairs was terminated at the end of a predetermined,fixed time period such that with the turntable running at its maximumspeed, the flow of electro-coating current through the respective brushpairs was terminated before full contact of the respective brush pairswas broken, so as to prevent sparking and arcing at the brush/segmentinterface.

As a consequence of that form of current control, whenever the turntablewas operated at a speed lower than the maximum value, theelectro-coating current flow ceased in advance of the instant ofbreaking full brush/segment contact, with the result that the intervalbetween the termination of one current pulse and the beginning of thenext current pulse in that cell increased with decrease in the turntablespeed.

This system of electro-coating current control (and of rejecting canshaving an unsatisfactory coating by means of a comparison against areference Coulomb count of the actual Coulomb quantity delivered) hasbeen found to be disadvantageous, since the deposit of electro-coatingmaterial was found to fall progressively with reduction in turntablespeed despite the fact that the duration and number of current pulsespassing through each cell remained constant. This is illustrated in theFIG. 20, where the graphs (a) to (d) show the effect of increasing thecurrent pulse interval whilst maintaining the other pulse and cellparameter values constant.

In those FIGS., current pulses of constant magnitude and constantduration of 80 msecs were spaced apart by different pulse intervals of10, 40, 70 and 100 msecs respectively. As the pulse interval increasedin the different examples, the total charge delivered to theelectro-coating cell decreased from 11.87 to 9.7, 9.1, and 8.7 Coulombsrespectively, and the deposits of electro-coating material fell from 332mg to 272, 255 and 244 mg respectively. Those FIGS. were obtained duringthe coating of an epoxy based material on to 33 centilitre DWI (drawnand wall-ironed) tinplate beverage cans.

Those FIGS. show (a) that during each current pulse the currentmagnitude falls progressively as the resistance of the film of depositedmaterial increases due to the progressively increasing and thickeningcoverage of the can body; and (b) that with short intervals, the rise ofcurrent at the start of each pulse is relatively rapid, whereas withlonger intervals, that rise of current is relatively slow. This slowerrise in current is believed to result from an increase in the resistanceof the deposited film during the interpulse period: the longer thatperiod, the greater the change in resistance.

Likewise, it is believed that whereas during the passage of a currentpulse the film of material deposited on the can body surface remainsfairly "open" so as to allow movement of the ionic species ofdissociated water and gas (O₂), during the current interval somerearrangement takes place within the deposited film so that a moreclosely-packed and hence more resistive film is formed: the greater thepulse interval, the more closely packed the film becomes. Hence, it hasbecome clear to us that the pulse interval should be kept as small aspossible.

Furthermore, it is believed that the phenomenon described above appliedequally in respect of both anodically and cathodically depositedmaterials (e.g. acrylic, polyester, epoxy-acrylic types ofelectro-coating materials), applied to a variety of different substrates(e.g. aluminium, steel, tinplate), at temperatures up to 30° C., andusing voltages up to 250 volts.

Thus, as has been referred to in the above-mentioned fourth aspect ofthe present invention, no such fixed duration of the current pulses isemployed, but instead, each current pulse is allowed to continue, afterfull brush/segment contact has been established, until that fullbrush/segment contact is about to be interrupted. One convenient way ofachieving that is to employ position sensing means arranged to detectthe angular position of the turntable relative to a datum, and toproduce switching-on control signal pulses at the instants when fullbrush/segment contacts have just been established, and switching-offcontrol signals when the continuance of those full contacts is about tocease. Such a sensing means may comprise, for example, two sensorsarranged to detect in succession the passage of each one of a series ofnotches spaced around the turntable at the angular pitch of theelectro-coating cells.

Thus, with such arrangements according to the present invention, thepulse duration is always at the maximum value possible, and the pulseinterval is always at the minimum value possible, for the particularturntable speed, since the whole period of full brush/segment contact isutilised; and the ratio of the pulse duration to the pulse interval isalways constant, regardless of the turntable speed. Since the pulseduration is now dependent on the turntable speed, and falls withincrease in that speed, the desired deposit of electro-coating materialwill occur in a different spread of time periods according to thatspeed.

Thus, according to another feature of this fourth aspect of the presentinvention, (a) the amounts of electrical charge (Coulombs) delivered toeach individual cell at the respective stations are measured andsummated to produce a control signal which is representative of thetotal charge delivered so far to that cell by the respective currentsthat have already passed through it; (b) that signal is comparedrepeatedly with a predetermined reference signal representative of thedesired deposit of electro-coating material; and (c) an inhibit signalis produced whenever that total charge signal exceeds the referencesignal, which inhibit signal is used to inhibit the delivery of furthercurrent pulses to that cell as it passes through subsequent stations.

By this means, the deposit of a desired amount of material issuccessfully achieved regardless of the turntable speed, since at lowerspeeds, the longer duration pulses cause an earlier production of aninhibit signal, after but a few long duration pulses, whilst at higherspeeds the shorter duration of the current pulses necessitates a largernumber of pulses in order to deposit the same amount of material.Furthermore, more precise control of the amount of material deposited ispossible since the termination of current flow in any particular cell isinitiated at a station at which the deposit has been completed.

Referring now to the schematic diagrams shown in the FIGS. 18 and 19.

FIGS. 18 shows the main circuits (including those of the power supplyunit 300 and the control and monitoring apparatus 302) for supplying andcontrolling the electro-coating current flows through the respectivecells of the apparatus. The respective rings 272 to 276 of slip-ringsegments 278 are represented there for simplicity's sake by the parallelstraight lines of dashes 700*, 702*, 704*, each dash representing onesegment 278, and the cooperating brushes 286 are shown adjacent thoselines of dashes.

The electro-coating direct current pulses are derived from a three-phasea.c. supply source 706* via a variable voltage transformer 708* and ahybrid thyristor rectifier bridge circuit 710* having its negativeterminal 712* connected to earth, and its positive terminal 714*connected to twelve similar parallel-connected electro-coating circuits716*.

Each such circuit 716 comprises, in serial connection, a currentlimiting/short-circuit detecting resistor 718* having a centre tappingthereof connected via a "crowbar" cell-short-circuiting thyristor (SCR)720* to earth, a brush 286 contacting the ring of slip-ring segments 276(connected to the respective cans 128), the can 128 and the internalelectrode 148 of an electro-coating cell 96 enclosing the can, a bursh286 contacting the ring of slip-ring segments 272 (connected with theinternal electrodes 148 of the cells), an electronic selector 722*constituted by a thyristor, a d.c. current transformer (DCCT) 724*, andan earth return connection 726*.

A group of three brushes 286 which make contact with the ring ofslip-ring segments 271 connected with the respective cell externalelectrodes 176, 123 are each connected to an earth connection 726through respective similar circuits 728*, each of which likewiseincludes in series an electronic selector switch 730* constituted by athyristor and a d.c. current transformer 732*.

The DCCTs 724, 732 supply current signals to respective "Coulombprocessing" means 734*, 736*, each of which incorporates an integratingmeans (not shown) for integrating (with respect to time) the associatedDCCT output signal so as to produce as its output signal a digital,"Coulomb" signal representing the charge delivered by theelectro-coating current flowing in the associated cell electrode system.

The magnitude of the electro-coating currents supplied to the cells iscontrolled by adjustment of the output voltage of the variable voltagetransformer 708.

The thyristor rectifier bridge 710 is controlled in an ON/OFF manner bya bridge control circuit 738* which receives control signals via acontrol circuit 740*.

Energisation of the respective electrode systems in the respective cellsis achieved by selective energisation of the selector thyristor switches722, 730, as determined by the presence/absence of firing controlsignals at the respective output circuits 742*, 744* of a cell selectingcontrol circuit 746*.

The control circuit 746 derives its input control signals from theoutput circuits of the respective shift registers 414 to 422, whichregisters derive input signals data from the "Coulomb processing"means/package 734/736.

Other data is supplied to those registers via an "over-current andcrowbar signal processing" means/package 748* which receives inputsignals from the "Coulomb processing" means/package 734, 736, and from a"crowbar" control circuit 750*. That processing means 748 determinesfrom the signals and data supplied thereto, and supplies to the shiftregister 414, the identity of a cell in which an over-current and/or ashort circuit has developed.

The crowbar thyristors 720 are controlled by a crowbar control circuit750* which derives its respective input signals, indicative of excessivemagnitudes of the respective electro-coating currents, from respectivetappings 752* on the respective current limiting resistors 718.

In the schematic diagram of the FIG. 19, the power and control circuitsof FIG. 18 are shown in a different format and in conjunction with (a) arepresentation, in plan view, of the rotatable turntable 14 and itsassociated electro-coating cells 96 and control devices.

The digital data shift registers and other data processing meansreferred to above may be incorporated in a microprocessor of anysuitable conventional kind, for example, a processor known as a "MAC 85"processor, and the respective processing means/packages may beconstituted as any appropriate combination of hardware and softwaremeans. Wherever appropriate, the various items shown in this Figure bearthe references assigned to them earlier in this description.

The reader's attention is hereby directed to the followingconcurrently-filed, co-pending patent applications which claim otheraspects of the disclosure of this application: No. 07/193,451, filed May6, 1988, No. 07/193,452 filed May 6, 1988, No. 07/193,455 filed May 6,1988.

We claim:
 1. Electro-coating apparatus for electro-coating containerscomprising:(a) a rotatable turret; (b) a plurality of electro-coatingcells carried on said turret at positions spaced circumferentially atconstant angular pitch, each cell incorporating at least oneelectro-coating electrode and being arranged to enclose a container inspaced relationship with said electrode and with an electro-coatingfluid therebetween whereby a flow of electro-coating current between theelectrode and an opposed surface of a container causes theelectro-coating of the surface; (c) a segmented slip-ring carried onsaid turret for rotation therewith, said slip-ring having successivesegments connected electrically with the respective electrodes ofsuccessive cells; (d) a plurality of stationary electrical brushesspaced around said segmented slip-ring and electrically contactingsuccessive segments of said segmented slip-ring at correspondingpositions on said segments; (e) a plurality of switching means connectedwith the respective brushes for energizing said brushes with anelectro-coating potential as required thereby to cause anelectro-coating current to flow in the respective cells; and (f) controlmeans for causing operation of said switching means thereby toalternately energize and de-energize said brushes; (g) said controlmeans including (i) a control disc arranged for synchronous rotationwith said turret; (ii) said control disc carrying a plurality of triggerdevices at positions spaced circumferentially therearound; (iii)stationary sensing means being disposed adjacent said control disc andarranged to sense the passage thereby of successive trigger devices assaid turret rotates and to provide in response to successive triggerdevices successive output signals for controlling said switching meansalternately in opposite senses thereby to successively energize andde-energize said brushes; and (iv) said trigger devices being positionedon said control disc so that each brush is energized with said potentialonly while it lies wholly in contact with a segment, and with each brushremaining energized for a predetermined constant angle of rotation ofsaid turret, corresponding to a predetermined high proportion of thecircumferential pitch of said segments, regardless of the speed ofrotation of the turret.
 2. Electro-coating apparatus according to claim1 wherein said trigger devices are positioned so that during turretrotation through each angular pitch each brush is de-energized for onlythe minimum possible part of said angular pitch which is necessary forthe avoidance of arcing and sparking between said brush and a saidsegment from which it is breaking contact, thereby to ensureenergization of each said brush for the maximum possible part of eachsaid angular pitch of turret rotation.
 3. Electro-coating apparatusaccording to claim 1 wherein said control disc is circular, and saidtrigger devices are constituted by discontinuities in the periphery ofsaid circular control disc.
 4. Electro-coating apparatus according toclaim 3 wherein said discontinuities comprise notches formed in saidperiphery of said control disc.
 5. Electro-coating apparatus accordingto claim 3 wherein said sensing means comprises proximity sensing meansfor sensing the proximity of the periphery of said control disc. 6.Electro-coating apparatus according to claim 4 wherein said sensingmeans comprises two proximity sensing devices disposed so as to senserespectively the beginning and the end of each said notch as it passesthereby.
 7. Electro-coating apparatus according to claim 6 wherein saidsensing devices sense respectively the beginning and the end of the samenotch simultaneously.
 8. Electro-coating apparatus according to claim 1including:(a) in each cell at least a second electro-coating electrodefor electro-coating a second surface of each container; (b) a secondsegmented slip-ring carried on said turret for rotation therewith, andcomprising successive second segments connected electrically with therespective second electrodes of successive cells; (c) a plurality ofstationary second electrical brushes spaced around said second segmentedslip-ring and electrically contacting successive second segments of saidsecond segmented slip-ring at corresponding positions on said secondsegments; and (d) a plurality of second switching means connected withthe respective second brushes for energizing said second brushes with anelectro-coating potential as required thereby to cause anelectro-coating current to flow in the respective second electrodes ofthe respective cells; and said control means is constructed and arrangedfor causing operation of said second switching means in like manner asthe first-mentioned switching means, thereby to alternately energize andde-energize said second brushes in like manner as the first-mentionedbrushes.
 9. Electro-coating apparatus according to claim 8 wherein saidtrigger devices are positioned so that during turret rotation througheach said angular pitch each said second brush is de-energized for onlythe minimum possible part of said angular pitch which is necessary forthe avoidance of arcing and sparking between said second brush and asaid second segment from which it is breaking contact, thereby to ensureenergization of each said second brush for the maximum possible part ofeach said angular pitch of turret rotation.
 10. Electro-coatingapparatus according to claim 8 wherein said control disc is circular,and said trigger devices are constituted by discontinuities in theperiphery of said circular control disc.
 11. Electro-coating apparatusaccording to claim 10 wherein said discontinuities comprise notchesformed in said periphery of said control disc.
 12. Electro-coatingappartus according to claim 10 wherein said sensing means comprisesproximity sensing means for sensing the proximity of the periphery ofsaid control disc.
 13. Electro-coating apparatus according to claim 11wherein said sensing means comprises two proximity sensing devicesdisposed so as to sense respectively the beginning and the end of eachsaid notch as it passes thereby.
 14. Electro-coating apparatus accordingto claim 13 wherein said sensing devices sense respectively thebeginning and the end of the same notch simultaneously. 15.Electro-coating apparatus according to claim 1 including:(a) in eachcell a third electro-coating electrode for electrically contacting saidcontainer thereby providing an electrical return path for eachelectro-coating current; (b) a third segmented slip-ring carried on saidturret for rotation therewith, and having successive third segmentsconnected electrically with the respective third electrodes ofsuccessive cells; (c) a plurality of stationary third electrical brushesspaced around said third segmented slip-ring and electrically contactingsuccessive third segments of said third segmented slip-ring atcorresponding positions on said third segments; and (d) a plurality ofelectric return circuit connections connected with the respective thirdbrushes for providing electrical return circuits for the respectiveelectro-coating currents flowing in the respective electrodes of therespective cells.
 16. Electro-coating apparatus according to claim 8including:(a) in each cell a third electro-coating electrode forelectrically contacting said container thereby providing an electricalreturn path for each electro-coating current; (b) a third segmentedslip-ring carried on said turret for rotation therewith, and havingsuccessive third segments connected electrically with the respectivethird electrodes of successive cells; (c) a plurality of stationarythird electrical brushes spaced around said third segmented slip-ringand electrically contacting successive third segments of said thirdsegmented slip-ring at corresponding positions on said third segments;and (d) a plurality of electric return circuit connections connectedwith the respective third brushes for providing electrical returncircuits for the respective electro-coating currents flowing in therespective electrodes of the respective cells.
 17. Electro-coatingapparatus according to claim 1 including a plurality of selector switchmeans connected respectively in serial manner with the respectiveswitching means, each selector switch means being operative forresponding to an electrical selection signal received thereby toselectively connect the associated brush for energization by saidelectro-coating potential on closure of the associated said switchingmeans.
 18. Electro-coating apparatus according to claim 1 wherein ineach cell first electrode is arranged to be received inside a containerfor depositing when energized electro-coating material on an internalsurface of the container.
 19. Electro-coating apparatus according toclaim 8 wherein in each cell second electrode is arranged to engagearound a container for depositing when energized electro-coatingmaterial on an external surface of the container.
 20. Electro-coatingapparatus according to claim 8 including a plurality of selector switchmeans connected respectively in serial manner with the respectiveswitching means, each selector switch means being operative forresponding to an electrical selection signal received thereby toselectively connect the associated brush for energization by saidelectro-coating potential on closure of the associated said switchingmeans, and wherein (a) in each cell said first electrode is arranged tobe received inside a container for depositing when energizedelectro-coating material on the internal surface of the container andsaid second electrode is arranged to engage around the container fordepositing when energized electro-coating material on an externalsurface of the container, and (b) said control means includes means fordelaying in respect of each said cell the selection and successiveenergizations of the associated second segment relative to selection andenergization of the associated first segment thereby to delay depositionof electro-coating material on the external surface of a container untilafter a predetermined plurality of successive depositions of suchmaterial have been made on the internal surface of a container bysuccessive energizations of said first electrode.
 21. Electro-coatingapparatus according to claim 20 including (a) means for summating inrespect of each cell the respective amounts of electro-coating chargedelivered to the cell through successive first brushes, (b) means forcomparing from time to time the total amount of charge so far deliveredto the cell through said first brushes against a preset reference value,and (c) means for inhibiting the delivery of further charge to the cellthrough further such first brushes whenever that total amount of chargeexceeds said preset value.
 22. Electro-coating apparatus according toclaim 20 including (a) means for summating in respect of each cell therespective amounts of electro-coating charge delivered to the cellthrough successive second brushes, (b) means for comparing from time totime the total amount of charge so far delivered to the cell throughsaid second brushes against a preset reference value, and (c) means forinhibiting the delivery of further charge to the cell through furthersuch second brushes whenever that total amount of charge exceeds saidpreset value.
 23. Electro-coating apparatus according to claim 21wherein the amounts of electro-coating charge delivered successively toa cell through successive first brushes are stored in digital form insuccessive stages of a digital data shift register, and are advancedthrough and accumulated in successive stages of the register as theassociated first segment advances from one said first brush to the next.24. Electro-coating apparatus according to claim 22 wherein the amountsof electro-coating charge delivered successively to a cell throughsuccessive second brushes are stored in digital form in successivestages of a second digital data shift register, and are advanced throughand accummulated in successive stages of that register as the associatedsecond segment advances from one said second brush to the next. 25.Electro-coating apparatus according to claim 8 including a plurality ofselector switch means connected respectively in serial manner with therespective switching means, each selector switch means being operativefor responding to an electrical selection signal received thereby toselectively connect the associated brush for energization by saidelectro-coating potential on closure of the associated said switchingmeans.
 26. Electro-coating apparatus according to claim 15 including aplurality of selector switch means connected respectively in serialmanner with the respective switching means, each selector switch meansbeing operative for responding to an electrical selection signalreceived thereby to selectively connect the associated brush forenergization by said electro-coating potential on closure of theassociated said switching means.